Every year, earthquakes around the world are responsible for the loss of thousands of lives and result in billions of dollars of structural damage, both directly and indirectly, from collateral damage aftermath. Earthquake events, as well as the related damage and losses caused thereby have increased in frequency and magnitude in recent years. For example, in the 1989 Loma Prieta earthquake that devastated portions of the San Francisco Bay Area, much of the damage was caused by systems failures after the earthquake hit. Compounding the direct damages from the actual earthquake, significant property loss resulted from gas-line ruptures and subsequent gas fires, electrical fires and inaccessible water reserves to name just a few. In many cases, emergency vehicles were unable to respond to these crises due to being stranded behind jammed garage doors and gates, the result of structural damage to their buildings.
The magnitude of an earthquake is measured in terms of a Richter scale value. Introduced in 1935 by Charles F. Richter, the Richter scale is a numerical scale for quantifying earthquake magnitude—typically it refers to local magnitude, but for larger quakes, it often refers to surface-wave magnitude. (Currently, large quakes are generally assigned a moment magnitude, which is scaled to be similar, but is based on seismic moment, and a better measure of the energy of an earthquake.) Since the Richter scale is logarithmic, very small earthquakes (microearthquakes) can have a negative magnitude. While the scale has no theoretical upper limit, the practical upper limit, given the strength of materials in the crust, is just below 9 for local or surface-wave magnitudes (and just below 10 for moment magnitudes).
It is well known that when an earthquake occurs, three sets of waves emanate from the point of origin: P (primary), S (shear) and R (Rayleigh). (There are also Love waves; a shear surface wave in addition to the S-wave, shear bulk wave. The speed of the Love waves is intermediate between S and R-waves). The “P-wave”, which is non-destructive and imperceptible to humans, is mainly a vertical motion wave that travels faster than the destructive S- and R-waves. More specifically, the P-wave is a compressional body wave; particle movement is parallel to the direction of propagation of the wave. Its speed is 5.5 to 7.2 km/sec in the crust and 7.8 to 8.5 km/sec in the upper mantle. Since P-waves travel about twice as fast as the S waves, they will arrive sooner. The greater the distance from the hypocenter of an earthquake one is, the greater the time differential between the arrival of P- and S-waves. On the West Coast of North America, for example, the speed of travel of the P-waves is approximately 6.2 miles per second. Therefore, if an earthquake were to occur at a depth of approximately 10 miles, and the epicenter was a distance of approximately 50 miles from the detector, an 8 second warning would be possible. It is apparent, of course, that if the earthquake were substantially deeper, larger in terms of magnitude and further away, an even longer warning time would occur. Depending upon the distance from the point of origin, a typical warning on the order of 1 to 25 seconds is possible.
Generally speaking, the P-waves have a natural frequency of approximately 5 Hertz (Hz) while S-waves have a frequency significantly less than the P-waves. The S-waves have a significantly larger amplitude than the P-waves and therefore are the waves that are principally involved in the destruction to structures. As indicated above, P-waves typically travel at a faster rate from the hypocenter to a given locale in comparison with S-waves. Thus, detection of P-waves can provide an early warning of the impending arrival of S-waves at a given location distant from the epicenter.
One of the primary difficulties in earthquake detection relates to the time factor involved in detecting tile P-waves. As will be realized, if P-waves can be detected as early as possible, this provides time for evacuation, etc., of a building or area in order to avoid potential human injury caused by the arrival of S-waves which, as indicated above, are the chief destructive waves transmitted by geological formations.
Devices and systems that provide advance warning of destructive earthquakes by detecting P-waves (the non-destructive primary earthquake waves) are disclosed in U.S. Pat. No. 5,760,696 entitled “Discriminating Earthquake Detector” and in U.S. Pat. No. 6,356,204 entitled “Method and Apparatus For Detecting Impending Earthquakes”. Based on advanced sensing technology, these devices can sound an alarm and/or activate Automatic System Response (ASR), thereby minimizing loss of life and property damage.
The prior art also includes various detectors and other arrangements to measure P-waves as a precursor to following S-waves. However, using existing arrangements, it has been difficult to detect P-waves at a distance from the epicenter of an earthquake without incurring large costs. A further problem is that it is often difficult to resolve false alarms from a real earthquake, due to interference in the instrumentation by extraneous local vibrations or other frequencies. It is desirable to provide a detector capable of discriminating between P-waves and ordinary, everyday ground and building tremors unrelated to an earthquake. In particular, detectors mounted to a building should be capable of discriminating between the natural vibration frequencies of the building structure, which are a function of the structure, and frequencies indicative of P-waves. This may be accomplished by means of an information processing unit that stores vibration data and is programmed to discriminate between frequently occurring frequencies and non-regularly occurring frequencies within the range of P-waves.
Typical of the art that has been patented in this field is the to Windisch, U.S. Pat. No. 4,689,997. The reference provides a detector that primarily employs a vertical spring barb mounted on a support. A coupler is supported on the other end of the barb and this coupler is connected through a coil spring to a mass positioned in concentricity with the barb and coupler. The spring and mass components are selected to have a natural resonant frequency corresponding to that of an earthquake tremor or other vibration to be detected. A switching circuit is provided to detonate an alarm once the earthquake frequency is detected. Windisch does not provide an integrated circuit mechanism for detection of earth tremors, but rather relies on a mechanical arrangement in the form of a spring and mass system. As is known, such systems are susceptible to temperature fluctuations that can alter the point at which the apparatus can detect the earthquake frequency, and are often delicate and thus difficult or more expensive to install in large volume. Further, the Windisch arrangement does not appear to provide a system that discriminates between simple extraneous vibration and earthquake caliber frequencies.
Caillat et al., in U.S. Pat. No. 5,101,195, provide a discriminating earthquake detector. The arrangement relies on an electromechanical combination having a cantilevered device with a predetermined mass on one end. During movement of the beam, an electrical signal is generated which, in turn, is useful for detection of P- and S-waves. Similar to the above-mentioned detectors in the prior art, the arrangement provided in this reference would appear to have limited utility in that there is no provision for a comparison between earthquake caliber waves and those which are simply extraneous, such as would be encountered as a result of traffic vibration, mechanical vibration in a building, aircraft vibration, etc.
U.S. Pat. No. 5,001,466, issued Mar. 19, 1991 to Orlinsky et al., provides an earthquake detector employing an electrically conductive liquid switch means among other variations thereof.
However, there remains a need for a highly accurate, relatively inexpensive detector for accurately measuring P-waves and generating a signal which can be used to drive a variety of types of annunciators and actuators.
Briefly, a presently preferred embodiment of the invention includes a printed circuit board having mounted thereon from one to three orthogonally disposed miniature piezo-electric sensors that function in a cantilever mode as inertia monitoring devices, a plurality of amplifying and filtering circuits for amplifying and filtering the outputs generated by the piezo-electric sensors, and a central processing unit responsive to the amplified signals and operative to generate output signals which can be used to drive optical and audible annunciators and device actuating systems. The sensors are formed by a thin piezo-electric film sandwiched between to metallization layers and are carried by a small rectangular sheet of polyester having one edge mounted to a PC board. A small mass is attached near the end of the cantilever, improving the inertia sensing capability of the system.
Among the advantages of the present invention is that it includes a small detector which consumes extremely little power.
Another advantage of the present invention is that it provides a relatively low-cost sensor that can be placed in multiple locations at reasonable expense.
Still another advantage of the present invention is that it provides highly accurate detection of earthquake related primary wave (P-wave) motion and generates an output that can be transmitted to remote locations as part of a system dedicated to announcing the impending arrival of an earthquake.
These and other objects and advantages of the present invention will no doubt become apparent to those skilled in the art after having read the following detailed description of the preferred embodiments illustrated in the several figures of the drawing.